The present invention concerns a press roll for use in a roll press of a paper machine for the treatment of web type material. More particularly, such a press roll is arranged together with a backing roll to form a press gap therebetween, with the deflection of the backing roll being controllable. The press roll comprises a tubular cylinder having a wall thickness amounting to up to one-eighth of the roll diameter. Each roll end includes a cylinder head with a journal.
Press rolls of the above type are known and, by and large, also have proved themselves to be acceptable in performance. Their specific purpose, in interaction with a backing roll of controllable deflection, is to treat material, which generally is continuous and web-shaped, in a specific way. In connection with paper machines, press rolls of the said and known type are used, e.g., in so-called press stations, for instance in a wet press or in calendering stands
As the width of the paper machines and also their operating speed increases, the problem of maintaining a constant line force across the entire web width assumes decisive significance.
In the operation of a roll press for the treatment of continuous web type material, it has been shown (compare Wochenblatt fuer Papierfabrikation, 11/12, 1988, page 490) that the tubular cylinder of a press roll deflects under the influence of the line force in various ways, depending on its wall thickness.
In one way, the flexure occurs as a beam deflection resulting from the support of the tubular cylinder by way of the journals of the cylinder heads in a pillow block. In another way, a flattening (or ovalization) of the round cross section of the tube occurs on the tubular cylinder, due to the radially effected load in the press plane between press roll and backing roll.
The said flattening of the tubular cylinder, also called shell deflection, is uneven and non-uniform across the width of the roll press. On its ends, the tubular cylinder is reinforced by the roll head in such a way that no flattening will occur there. However, with increasing distance from the heads, i.e., from the support bearing, the support effect of the head decreases with thin-walled tubular cylinders. The said support effect approaches "zero" in the center area of the roll press. The flattening, i.e., the radial deformation of the tubular cylinder is noteworthy especially when the wall thickness is small as compared to the roll diameter, for instance one-eighth thereof or less.
While the flex line of a beam, due to the line load of the backing roll, shows a pattern that is simple to compute, the shell deformation of the tubular cylinder across the width of the roll press can be calculated only with difficulty. This shell deflection of the tubular cylinder can be determined only at considerable computation expense making use of the so-called "finite elements" calculation. It has been demonstrated that the patterns of shell deflection and beam deflection vary, viewed across the width of the roll press.
If a roll press is comprised of a backing roll with controllable deflection and a plain press roll with a relatively thin-walled tubular cylinder, the beam deflection of the press roll can be extensively compensated for by way of controlling the deflection of the backing roll. So far it has not been possible to correct also the radial deformation, i.e., the deflection caused by shell deflection, or to compensate for it. Thus, impermissible non-uniformities remain in the line force distribution, which results in impermissible quality fluctuations in the web type material.
Basically, it would be conceivable to use instead of the plain press roll a second roll with controllable deflection. In view of the additional expense, however, this proposed solution is unrealistic.
For minimization of the error resulting from the shell deflection it is known to dimension the wall thickness of the tubular cylinder so large that the shell deflection relative to the overall deflection of the press roll will be negligible. However, this leads to an expensive and heavy roll.
The problem underlying the invention is to provide a press roll of the first mentioned type that has a relatively thin-walled tubular cylinder and which is therefore lightweight, but which in being pressed onto a backing roll with controllable deflection nonetheless has a flex line which maximally approximates that of the backing roll. It is meant by a "relatively thin-walled tubular cylinder" one where the wall thickness amounts to 1/8.sup.th to 1/25.sup.th of the outside diameter.